Very First Tank Upgrade.

Upgrading from 80 litres (roughly 22 gallons, I think) to 180 litres (roughly 48 gallons, I think).
How do I do this without stressing the fish and disrupting the parameters? Apprehensive because they have been living in perfect conditions for 6 months so far but I want them to have more room.
I have two fancy goldfish.
Thank you.

It won't be difficult . As long as you transfer your media into your new filter (or if that isn't possible, you can run them at the same time until the new filter is seeded) you'll be insta-cycled.

Imagine it like a large WC; feel free to use all new water in the tank, or mix some clean old tank water (no substrate vac!) with the new water to "mimic" a WC if your goldfish aren't used to very large WC. Just make sure to match the parameters as closely as you can, as if you were doing a WC (sorry, I sound like a broken record, but that's the best analogy I can compare to )

You can bag them up or float them in a large enough container to acclimate to the temp; you can also do a small acclimation by adding some new tank water to their containers every 15 min or so if you are worried. But if the parameters are very similar/identical, you can just float to match the temp and go ahead and release them.

Take some established media from the old tank and put it in the new filter, and let the tank cycle. Then do a 25% water change and replace the water with water from the old tank. To transfer the the fish I would not use a bag because that could be more stressful for such a large fish. Fill a large floatable plastic container and fill it with the old tank water put the fish in there and float it in the new tank with the lights and filter off for about 20 minutes. This is just the way that works for me and there are probably more that work just as well or even better.

It won't be difficult . As long as you transfer your media into your new filter (or if that isn't possible, you can run them at the same time until the new filter is seeded) you'll be insta-cycled.

Imagine it like a large WC; feel free to use all new water in the tank, or mix some clean old tank water (no substrate vac!) with the new water to "mimic" a WC if your goldfish aren't used to very large WC. Just make sure to match the parameters as closely as you can, as if you were doing a WC (sorry, I sound like a broken record, but that's the best analogy I can compare to )

You can bag them up or float them in a large enough container to acclimate to the temp; you can also do a small acclimation by adding some new tank water to their containers every 15 min or so if you are worried. But if the parameters are very similar/identical, you can just float to match the temp and go ahead and release them.